Temperature only tells half the story. When factored with the relative humidity, the result is called the Heat Index, which describes how hot it feels outside. We were surprised to learn that there's only a few hours during the day that aren't under a cautionary flag.
The Heat Index is the one number you should listen for in weather reports more than any other if you are going to be outside and engaging in physical exertion whether for fun, exercise or work. If the numbers reach a dangerous level mid-morning and last throughout the evening, you may want to reschedule your physical activity to the earliest possible time in the morning.
If you look over the numbers below*, you'll see how early in the day the heat index climbs past 80 degrees. In addition, once the heat arrives for the day, it tends to stick around all afternoon and into the evening, usually not dissipating until early morning.
* The shaded values reflect degrees Farenheit. The heat index data was recorded and reported by Wunderground.com on August 5, 2006.Exposure to full sunshine can increase heat index values by up to 15 °F.
Heatstroke possible with only brief activityHeat cramps, exhaustion, headaches after brief activityHeat cramps, exhaustion, headaches after prolonged activityExercise causes fatigue more rapidly.
Switching your physical activity to early morning can have additional benefits as well. Once the sun comes up on clear days, the Ultraviolet Index also rises, increasing the risk of sunburn and other ill effects of ultraviolet light. As the summer months fall within the Ozone Season, further benefits can be gained by exercising early before ozone levels reach alert status, causing respiratory irritation and inflammation in much of the population.
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